1. Field of the Invention
The invention relates to a method and apparatus for producing high-frequency oscillations. More specifically, the invention relates to a method and apparatus for producing such oscillations without the drawbacks of prior high-frequency oscillators.
2. Related Art
Almost every electronic instrument contains an oscillator or waveform generator of some type. Such devices as signal generators, function generators and pulse generators clearly require a source of regular oscillations. However, perhaps less obviously, any cyclical measuring instrument or other instrument that initiates measurements or processes or whose function involves periodic states or waveforms also requires such a source. Such devices may include networking equipment, wireless equipment, computers and other applications. In order to allow these devices to operate at higher and higher frequencies, the frequency of the waveform sources must increase as well.
It is desirable to use oscillators that maximize noise performance while operating with minimum power consumption, and that cost as little as possible (i.e., take up the least amount of die area). However, at very high frequencies, oscillators may suffer from a number of problems, including:                1. Resonator losses. Metalization and via losses may lead to increased power consumption for a given phase noise performance. For example, when using shorted microstrip transmission lines, a desirable approach at very high frequencies, such losses associated with the short become particularly troublesome and degrade the performance of the oscillator.        2. Layout difficulties. The layout of the oscillator circuit should be very compact at very high frequencies in order to minimize the influence of parasitic reactances on oscillator performance. Since the resonator is often much larger than the active circuitry, such a compact layout may be difficult to achieve, particularly when the two ports of a resonator are separated physically.        3. Noise cyclostationarity. Oscillators typically suffer from periodic moments of increased or decreased sensitivity to noise from external sources or from active devices in the oscillator circuit itself. It is highly desirable to refresh the energy stored in the resonator during such moments of decreased sensitivity in order to improve the performance of the oscillator with respect to phase noise and jitter. This approach requires accurate time-alignment of the active circuitry driving waveforms with oscillation voltages, which alignment becomes increasingly difficult as the frequency increases. Such an approach also requires a circuit architecture that is capable of generating the narrow current pulses that are applied to the resonator during the moments of reduced sensitivity.        4. Device delay. The unavoidable delay in active devices makes it difficult to optimally align the response of the active circuitry with the resonator waveforms. The resulting phase shift associated with this delay leads to an off-resonance oscillation and a corresponding reduction of the amplitude of the oscillation, and causes degraded phase noise performance. Device delay also reduces startup gain margin, possibly preventing startup entirely, and may also introduce unwanted parasitic oscillation modes.        
In addition, it is often desirable to cross-couple oscillators to generate quadrature signals for timing purposes. One such example is shown in U.S. Pat. No. 6,188,292 (see, e.g., FIG. 3). However, this approach uses variable coupling after the non-linear elements of the oscillators so that the frequency of the output may be varied, and this results in high phase noise that is undesirable.
These problems are overcome, at least in part, by the present invention.